The present invention relates generally to a bus in a communication system, and more particularly to a time division multiplexed bus.
Communications networks provide communications paths for voice and data using different protocols. In North America and other locations, a set of transmission signals referred to as the North American time division multiplexing hierarchy is used. This hierarchy includes DS1, DS2 and DS3 communications signals which are well-defined according to the following standards: ANSI T1.107, ANSI T1.403, Bellcore TR-TSY-000007, ATandT TR62411 and ATandT TR54016.
A DS1 signal has a transmission rate of 1.544 Mbps per second (Mbps). A DS2 signal includes four DS1 signals plus some overhead bits and has a transmission rate of 6.312 Mbps. A DS3 signal includes twenty-eight DS1 signals and has a transmission rate of 44.736 Mbps.
In other geographical areas, such as Europe, the time division multiplexing hierarchy described above is not used, but a European hierarchy with different transmission rates from that of the DS1, DS2 and DS3 is used. For example, an E1 signal with a rate of 2.048 Mbps that carries thirty channels is the lowest level of the multiplexing hierarchy, rather than a DS1 signal with a rate of 1.544 Mbps that carries twenty-four channels.
A standard called the synchronous optical network (SONET) protocol provides a common interface for transporting the different signaling hierarchies over an optical fiber. The SONET protocol can transport signals from both the North American and European hierarchies. The SONET standard defines a hierarchy of digital data rates, and is published in International Telecommunications Union (ITU) Recommendations G.707, G.708 and G.709. The SONET standards also include ANSI T1.105, ANSI T1.105.01, ANSI T1.105.02, ANSI T1.105.03, ANSI T1.105.03a, ANSI T1.105.03b, ANSI T1.105.04, ANSI T1.105.05, ANSI T1.105.06, ANSI T1.105.07, ANSI T1.105.07a and ANSI T1.105.09.
Each level of the SONET hierarchy is referred to as a synchronous transport signal (STS) level. The lowest level, STS-1, has a transmission rate of 51.84 Mbps. The STS-1 level can transport a single DS-3 signal or many lower rate signals, such as DS1 and DS2. Higher transmission rates are supported in the SONET hierarchy by combining multiple STS-1 signals into an STS-N signal. The SONET hierarchy ranges from the STS-1 level which has a transmission rate of 51.84 Mbps and a payload rate of 50.112 Mbps to an STS-48 level which has a transmission rate of 2,488.2 Mbps and a payload rate of 2,405.376 Mbps.
In FIG. 1, in a prior art system, a communications path is provided between an optical fiber 20 and facility lines 22 using a SONET transceiver 24 and SONET mappers 26. The SONET transceiver 24 provides the optical to electrical interface between the fiber 20 and the SONET mappers 26. In the SONET transceiver 24, a SONET transmitter 32 is electrically connected to an add bus 34 from the SONET mappers 26 and a SONET receiver 35 is electrically connected to a drop bus 38 of the SONET mappers 26.
The SONET mappers 26 transmit and receive facility signals on the facility lines 22. In one embodiment, the facility signals are electrical signals having a predetermined transmission rate. To transmit facility signals, the SONET mappers 26 map the facility signals to a SONET signal having a predefined format. The predefined format includes timeslots that are associated with each facility signal. The SONET mappers 26 also receive a SONET signal and map the SONET signal to the facility signals, using the predefined format, for transmission over the facility lines 22.
The add bus 34 and the drop bus 38 from each SONET mapper 26 are well-known Telecom bus interfaces. Mapping is performed according to ANSI standard T1.105 and Bellcore standard GR-253-CORE for T1.5 and International Telecommunications Union (ITU) G.709 for a synchronous mapping structure. Both the add bus 34 and the drop bus 38 of the SONET mappers 26 have an eight bit wide data path and use additional timing signals. Alternately, the add bus 34 and the drop bus 38 of the SONET mappers 26 have an nine bit wide data path that includes a parity bit and use additional timing signals.
As shown in FIG. 2A, the basic SONET building block is an STS-1 frame 40 which has a header 42 and a SONET payload envelope (SPE) 44. The frame 40 has 810 bytes (octets) and, is transmitted once every 125 xcexcsec. The frame 40 is typically viewed as a matrix having nine rows and ninety columns. Transmission is one row at a time from left to right and top to bottom. The header 42 includes overhead octets, and the SPE 44 carries data. The SPE 44 has eighty-seven columns of which forty-eight carry data and the remainder are overhead.
The SONET specification defines synchronous formats for the SPE for transmission rates below the STS-1 level. The STS-1 SPE 44 is subdivided into virtual tributaries in which each virtual tributary (VT) is associated with a signal having a predefined transmission rate. FIGS. 2B, 2C, 2D and 2E show each type of VT and the number of rows and columns associated with that VT. As shown in FIG. 2B, to transport DS1 signal, VT 1 uses nine rows and three columns of the SPE. Table 1 below summarizes the virtual tributaries, their bit rate and size.
A VT 1 has sufficient bandwidth to transmit a DS1 signal, while a VT 6 has sufficient bandwidth to transmit a DS2 signal. A VT 3 is not the same as a DS3 and has a much lower bit rate than the DS3. To transmit a DS3 signal, the entire STS-1 SPE is dedicated to the one DS3.
FIG. 2F shows four VT 1""s, labeled A, B, C and D. Each VT 1 has the capacity to transmit a DS1 signal. The VT 1""s are interleaved among themselves for transmission. FIG. 2G shows three VT 2""s, labeled X, Y and Z. The VT 2""s are interleaved among themselves for transmission. FIG. 2H shows two VT 3""s, labeled M and N. The VT 3""s are interleaved among themselves for transmission. FIG. 2I shows a VT 6, labeled O.
In FIGS. 2J and 2K, an exemplary format for an SPE 44 that transmits the four VT 1""s, the three VT 2""s, the two VT 3""s and the one VT 6 is shown. A complex interleaving pattern associates each VT with particular columns or timeslots. The VT label associated with a timeslot is indicated in each column and the timeslot number of the SPE 44 is shown below each column. Some of the columns that do not have an associated label contain pointer values and are used to compensate for timing variations. The interleaving patterns are defined in the ANSI T1.105 specification.
Many communication systems, such as switches and private branch exchanges, use time-division multiplexed buses. In computer telephony integration, commonly used internal system buses include the Multi-Vendor Interface Protocol (MVIP) and H.100A buses. The MVIP and H.100A buses are targeted towards connections that use integral multiples of sixty-four kilobits per second (Kbps). MVIP and H.100A also impose a maximum transmission speed of 16 Mbps. Typical transmission speeds encountered in communications networks are 1.544 Mbps (DS1), 44.736 Mbps (DS3) and 51.84 Mbps (SONET STS-1). Of the three aforementioned rates, DS3 and STS-1 exceed the maximum transmission speed of MVIP and H.100A by a factor of over two, and only 51.84 Mbps is a multiple of sixty-four Kbps.
Therefore, a time-division multiplexed bus for use internally in a communications system that supports transmission rates that are not integral rates of 64 Kbps is needed. The bus should also be capable of handling transmission speeds exceeding 16 Mbps, including 44.736 Mbps and 51.84 Mbps.
Another disadvantage of the MVIP and H.100 buses is that the MVIP and H.100 buses have an absolute clocking constraint which, under certain conditions, corrupts the data in the signal being transported. If the facility signal that is being transported across the MVIP or H.100 bus is based on timing which is slightly slower than that of the MVIP or H.100 bus, the circuitry responsible for transporting the facility signal is required to repeat data periodically to compensate for the difference. Conversely, if the facility signal being connected across the MVIP or H.100 bus is based on timing which is slightly faster than the MVIP or H.100 bus timing, the circuitry responsible for transporting the facility signal is required to delete data periodically to compensate for the difference. Both of these actions corrupt the facility signal being transported. Facility signals frequently have slight timing inaccuracies, and thus cannot be accurately transported across an H.100 or MVIP bus.
Therefore, there is a need for a time-division multiplexed bus for use internally in a communications system that does not corrupt the data in the transported facility signal when the timing of that facility signal deviates from the timing of the time-division multiplexed bus.
A SONET bus provides high speed interconnections using SONET mappers and a bidirectional drivers. A SONET bus has a set of SONET mappers that transmit and receive facility signals on facility lines. Each facility line operates at a predetermined speed. Each SONET mapper generates a SONET signal by mapping the facility signals received by the SONET mapper into a predefined format for transmission. The predefined format includes timeslots associated with each received facility signal. Each SONET mapper receives a SONET signal and maps the received SONET signal into the facility signals transmitted by the SONET mapper on the facility lines. Each SONET signal includes an associated set of the facility signals. At least one counter outputs a timeslot count signal for synchronizing the timeslots of the facility signals. A set of bidirectional drivers has a mapper side and a system side. Each bidirectional driver receives the timeslot count signal. A first set of interconnections separately couples each bidirectional driver at the mapper side to at least one SONET mapper of the set of SONET mappers, such that each bidirectional driver receives the SONET signal generated by at least one SONET mapper and transmits the SONET signal received by at least one SONET mapper. A second set of interconnections couples the bidirectional drivers to each other at the system side. The bidirectional drivers transmit one or more of the facility signals via the second set of interconnections by extracting the one or more facility signals from the SONET signals generated by the SONET mappers in accordance with the timeslot count signal and the predefined format.
In another aspect of the invention, multiple parallel SONET buses provide a high speed interconnection.
In yet another aspect of the invention, a SONET ring interconnects sets of multiple parallel SONET buses.
In an alternate aspect of the invention, a communications system uses the SONET bus of the present invention.
The SONET bus provides a flexible high speed interface among various components. In addition, the SONET bus is not limited to transmission speeds of multiples of sixty-four Kbps, and allows transmission of signals with rates exceeding sixteen Mbps. The SONET bus also compensates for timing variations of the facility signals with respect to a SONET signal without corrupting the data of the facility signal.